Demand hydraulic system with cooling flow circuit

ABSTRACT

In a hydraulic system having a variable displacement pump and one or more control valves, a control system is provided that achieves load-responsive control of the displacement of the pump when any control valve is actuated and further achieves a standby condition which includes low pump displacement, low standby pressure and a cooling flow path that circulates a small amount of fluid through the pump and to sump to prevent over-heating.

United States Patent 1 Miller et al.

1 DEMAND HYDRAULIC SYSTEM WITH COOLING FLOW CIRCUIT [76] Inventors: Wendell E. Miller, 1907 Crescent Dr Warsaw, 1nd. 46580; Kenneth G. McMillen, deceased, late of New Castle, lnd.; Leanne S. McMillen, administratrix, Fort Wayne, Ind,

[22] Filed: July 13, 1972 I21] Appl. No.: 271,301

[52] U.S. Cl. 60/456, 60/D1G. 5 [51] Int. Cl. FlSb 21/04 [58] Field of Search 60/468, 456, DIG. 5, 329

[56] References Cited UNITED STATES PATENTS 6/1959 Allen et al. 60/468 X Apr. 2, 1974 2,986,166 5/1961 Hocn 60/97 P X 3,597,921 8/1971 Mel-Burnett. 3,664,129 5/1972 Schwab 60/DIG. 5

Primary Examiner-Edgar W. Geoghegan Attorney, Agent, or Firm-Donald W. Banner [57] ABSTRACT In a hydraulic system having a variable displacement pump and one or more control valves, a control system is provided that achieves load-responsive control of the displacement of the pump when any control valve is actuated and further achieves a standby condition which includes low pump displacement, low standby pressure and a cooling flow path that circulates a small amount of fluid through the pump and to sump to prevent over-heating.

5 Claims, 3 Drawing Figures DEMAND HYDRAULIC SYSTEM WITH COOLING FLOW CIRCUIT SUMMARY OF THE INVENTION- .applied to the system.

Load responsive systems which utilize a variable displacement pump also have a low power loss during standby conditions becausethe displacement of the pump is reduced almost to zero and also because a low standby pressure is maintained. The present invention retains these desirable features of theload responsive system while adding a cooling circuit. This cooling circuit allows a small quantity of fluid to flow from the pump discharge port to a sump when the pump is operating in a standby condition and this cooling flow circuit is automatically blocked when any control valve is actuated.

The present invention also has the added advantage of allowing the slip fluid from the pump housing to be routed to the pump intake port for recirculation through the pump thereby elimin'ating the housing drain. In conventional systems, recirculating of this drain fluid causes overheating of the pump because the slip fluid from the housing drain contains the heat 1 caused by the pumps mechanical and hydraulic inefficiencies. In particular, the slip fluid includes heat caused by viscous shear of the fluid between thevalving surfaces and between the piston slippers and swash plate.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing a control circuit embodying the principles of the present invention;

FIG. 2 is a schematic view showing a control circuit embodying the principles of the present invention in a modified form; and

FIG. 3 is a diagrammatic view ofa control circuit according to another embodiment of the present invention showing certain of the valves in cross-section.

Referring to FIG. 1, a hydraulic control circuit comprising a variable displacement pump 10, a sump 11, a hydraulic motor 12, and a control valve for controlling the flow of fluid therebetween is shown. The variable displacement pump 10 has a swash plate 21 which controls the output of the pump. The swash plate 21 is controlled by displacement control mechanism 20. The displacement control mechanism includes a displacement increasing cylinder 22 and a spring 23 therein which biases the pump to full displacement. A displacement decreasing cylinder 24 is provided to act on the opposite end of the swash plate to move the pump to a minimum displacement. The control valve 30 is a simple two-position valve which, in one position, provides a variable orifice for passing fluid from the pump to the motor to actuate said motor, and in a neutral position, isolates the output of the pump 10 from sump 11. The control valve 30 in the actuating position provides a path to supply motor actuating pressure to the displacement control mechanism 20 to control the pump in a known manner as described in the aforementioned patent.

Conduit means are provided connecting the components of the hydraulic system. Conduit 40'connects the sump with the input side of the pump 10. Conduit 41 connects the output of the pump 10 to the control valve 30. A check valve 15 is provided in this conduitto prevent load pressure from the motor 12 from flowing back to the pump 10. Conduit 42 connects the control valve 30 to the motor 12. Conduit 43 connects the motor 12 to the sump ll. Conduit 44 connects the control valve 30 directly tothe sump ll. Conduit 45 connects the output side of the pump 10 to the displacement decreasing cylinder 24 of the displacement control mechanism 20. Conduit 46 connects the output of .the pump 10 with the displacement-increasingcylinder 22 of the displacement control mechanism 20. Conduit 46 is provided with an orifice 50 between the pump output and conduit 47. Conduit 47 is provided to connect conduit 46 and control valve 30. The control valve in its neutral position, as illustrated in FIGQl, connects conduit 47 to conduit 44 and thereby to the sump 11.

In operation, with the control valve 30 in neutral position, a cooling flow path is established and a limited quantity of fluid is passed through the orifice 50 and to the sump 11 via the control valve 30 and the branch conduit 47. This flow of a portion of the output of the pump through the cooling flow path provides for circulation through the system of enough fluid to carry off the heat generated by the-pump. I

The pump output fluid in excess of this limited flow to the sump, causes a buildup of fluid pressure in conduits 41 and 45 and in displacement-decreasing cylin der 24. At the same time, the fluid pressure in displacement-increasing cylinder 22 and in conduit 46 is reduced to the magnitude of the pressure in sump conduit 43 by the connection of conduits 46 and 47 with conduit 44 when the control valve is in the neutral position. Thus the buildup of fluid pressure in conduits 41 and 45 is effective to reduce the displacement of the pump; so'that, the pump produces only the amount of fluid that can be bypassed to sump through orifice 50 at the fluid pressure that is required to actuate swash plate 21 against the load of spring 23.

When the control valve 30 is moved to a motoractuating position, the pump output is fed to the motor 12 and the motor-actuating pressure is fed to the displacement control mechanism 20 to control the pump in a known manner.

It should be noted that when the control valve is in the actuating position, branch conduit 47, which provided the cooling flow to the sump 11, is no longer connected to the sump. Thus, in the disclosed system, when the work is being done, in other words the motor functioning, the cooling flow circuit is interrupted,

therefore there is no waste of this volume of flow in a working condition.

The embodiment shown in FIG. 2 is very similar to that of FIG. 1. The displacement control mechanism'20 is provided with a minimum displacement stop 25 whereby the displacement of the pump 10 cannot be adjusted to a zero displacement position. The orifice 50 of the FIG. 1 embodiment has been replaced by'a pilot relief valve 14. Operation of the embodiment of FIG. 2 is similar to that of FIG. 1. Since the pump 10 cannot be placed in a zero displacement condition because of the minimum displacement stop 25,. alimited flow will be forced past the pilotrelief valve 14 and through the control valve 30 to the sump 11 to provide a cooling flow of fluid. This embodiment retains the advantageous feature of interrupting the cooling flow path when in a working condition.

The FIG. 3 embodiment shows a pump having a displacement control mechanism comprised of a swash plate 21 spring biased to full displacement by spring 23, a displacement decreasing cylinder 24, and a sensor valve 60.

The sensor valve 60 comprises a valve spool 61 slidable in a'bore 62 provided in a valve body 63. The body 63 includes ports 64, 65, 66, 67 and 68. The valve spool includes lands 69, 70 and 71 and has a central bore 72 passing through it. The bore 72 is provided with an orifice 73. The ends of the valve body 63 are closed and a spring 75 is provided between the land 71 and the end of the valve body urging the valve spool 61 to the left. Movement of the spool 72 provides alternate communication between ports 65 and 66 or 66 and 67 via land 70.

The control valve of FIG. 3 is a four-way valve of known construction such as disclosed in U. S. Pat. No.

3,592,216 of common assignee. The structure of valve 30 will be obvious to one skilled in the art; since this valve has no bearing on the present invention, in will not be further described. Control valve 30 is shown connected for controlling a double-acting hydraulic motor 12. A port 32 in control valve 30 provides a pressure equal to that required by the load imposed on motor 12. l

Conduit 80 connects port 64 to the output of the pump 10. Conduit 81 connects port 65 to the output of the pump. Port 66 is connected to the displacement decreasing cylinder 24 by circuit 82. Port 67 is connected to the sump 11. Port 68 is connected to port 32 of the control valve 30 by conduit 83.

In operation of the hydraulic system of FIG. 3, the fluid pressure provided by the pump 10 is supplied to the control valve 30 by conduit 41. When the control valve 30 is in the neutral position, this fluid pressure is blocked by the control valve and acts upon the land 69 of-the sensor valve spool to move the spool to the right against spring 75. This movement causes communication between the ports 65 and 66, and pump pressure is provided through the conduit 81, ports 65 and 66, and conduit 82 to act on the displacement decreasing cylinder 24 and cause the pump 10 to decrease its displacement. Some fluid is allowed to pass through the central bore 72 of the valve spool 61 and the orifice 73 and out port 68 through conduit 83 to the sump 11 via the control valve 30. This limited fluid flow through the orifice 73 in the central bore 72 provides enough circulation in the hydraulic system to carry off'the heat built up by the pumping action. When control valve 30 is moved to an operating position, the pump 10 provides pressure fluid to the motor 12 and a pressure indicative of the load on the motor 12 is provided to the port 68 of the sensor valve 60 in a manner completely described in the aforementioned U. 8. Pat. No. 3,592,2l6. This pressure acts on the land 71 to move the valve spool 61 to the left and connect the displacement decreasing cylinder 24 to the sump via ports 66 and 67. The displacement of the pump 10 is then increased to that required by the load by the action of the displacement increasing spring 23.

When the control valve 30 is in an operative position (as described in the above-mentioned patent),-there is no connection to the sump via the port 32, therefore the cooling flow path described in the neutral position of the control valve 30 is interrupted in the operative position of the control valve.

Various features and several embodiments of the present invention have been shown and desribed. However, it should be obvious to one skilled in the art that modifications may be made therein without departing from the scope of the invention.

We claim:

1. A hydraulic system comprising a variable displacement pump, a motor, a control valve, and a sump;

adisplacement control mechanism operative to control the displacement of said pump in response to the load imposed on said motor;

said control valve having a neutral position in which said motor is isolated from said pump and said pump is connected through an orifice and said control valve to said sump and having a motor actuating position in which said pump is connected to said motor, the motor actuating pressure is connected to the displacement control mechanism, and said connection between said pump and said sump is blocked;

whereby in said neutral position of said control valve a limited flow of fluid is provided from said pump to said sump to carry off the heat generated thereby and in said motor actuating position said connection tosaid sump is interrupted.

2. A hydraulic system as claimed in claim 1 in which said displacement control mechanism includes a stop means which provides for at least a minimum pump output, and said orifice is comprised of a spring loaded check valve.

3. A hydraulic system as claimed in claim 1 in which said displacement control mechanism includes a sensor valve;

said sensor valve being comprised of a valve alternately providing communication between said pump output and said displacement control mechanism or said displacement control mechanism and said sump,

the control of said sensorvalving being provided by said motor actuating pressure.

4. A hydraulic system as claimed in claim 3 wherein said orifice is provided in said sensor valve.

5. In a hydraulic system having a variable displacement pump, a displacement control mechanism for said pump including a sensor valve, at least one control valve, a hydraulic motor for each control valve, and a sump;

each said control valve having a neutral position in which the pump output is isolated from said hydraulic motor and a limited flow of said pump out-' put is connected to said sump through said control valve,

said control valve having at least one hydraulic motor actuating position in which said connection of said pump to said sump is interrupted,

said sensor valve connected between said pump and said displacement control mechanism and adapted to control said displacement control mechanism in accordance with the load on said motor,

and said limited flow to said sump through said control valve in said neutral position provides a circulation of fluid in said system to cool said system and is routed through said sensor valve. 

1. A hydraulic system comprising a variable displacement pump, a motor, a control valve, and a sump; a displacement control mechanism operative to control the displacement of said pump in response to the load imposed on said motor; said control valve having a neutral position in which said motor is isolated fRom said pump and said pump is connected through an orifice and said control valve to said sump and having a motor actuating position in which said pump is connected to said motor, the motor actuating pressure is connected to the displacement control mechanism, and said connection between said pump and said sump is blocked; whereby in said neutral position of said control valve a limited flow of fluid is provided from said pump to said sump to carry off the heat generated thereby and in said motor actuating position said connection to said sump is interrupted.
 2. A hydraulic system as claimed in claim 1 in which said displacement control mechanism includes a stop means which provides for at least a minimum pump output, and said orifice is comprised of a spring loaded check valve.
 3. A hydraulic system as claimed in claim 1 in which said displacement control mechanism includes a sensor valve; said sensor valve being comprised of a valve alternately providing communication between said pump output and said displacement control mechanism or said displacement control mechanism and said sump, the control of said sensor valving being provided by said motor actuating pressure.
 4. A hydraulic system as claimed in claim 3 wherein said orifice is provided in said sensor valve.
 5. In a hydraulic system having a variable displacement pump, a displacement control mechanism for said pump including a sensor valve, at least one control valve, a hydraulic motor for each control valve, and a sump; each said control valve having a neutral position in which the pump output is isolated from said hydraulic motor and a limited flow of said pump output is connected to said sump through said control valve, said control valve having at least one hydraulic motor actuating position in which said connection of said pump to said sump is interrupted, said sensor valve connected between said pump and said displacement control mechanism and adapted to control said displacement control mechanism in accordance with the load on said motor, and said limited flow to said sump through said control valve in said neutral position provides a circulation of fluid in said system to cool said system and is routed through said sensor valve. 